1. Field of the Invention
This invention relates generally to patterning of polymers. More particularly, this invention relates to patterning of polymers in a high density interconnect process in such a way as to allow use of low dielectric constant polymers, and to remove any polymer ridges which may be formed around pattern openings.
2. Description of the Related Art
Materials such as TEFLON polytetrafluoroethylene (Teflon is a trademark of E.I. duPont de Nemours and Co.) and other fluorocarbon polymers have highly desirable properties in the manufacture of high density interconnect (HDI) circuits. Polytetrafluoroethylene has one of the lowest dielectric constants in the polymer field, optical clarity, and excellent chemical and temperature stability. However, laser patterning of fluorocarbon polymers is hindered because the various fluorocarbon polymer derivatives are transparent to light of wavelengths generally greater than 200 nm. Thus, the use of continuous wave (CW) scanning lasers for patterning and ablation that are in the 350-360 nm range have been ineffective.
Commonly assigned Cole et al., U.S. application Ser. No. 07/936,496, filed Aug. 28, 1992, entitled "Laser Ablatable Polymer Dielectrics and Methods," discloses a method of doping polyesters with a small amount of dye to make them absorb at the desired wavelength. Polytetrafluoroethylene, however, is not solventable (i.e., does not dissolve in any known solvent), and therefore this approach of doping with a molecule in a common solvent will not work. It is possible to heat polytetrafluoroethylene above its melting point and blend in absorbing dyes, but, because this melting would require processing at elevated temperatures (in excess of 275.degree. C.), and the dyes used are not thermally stable at these temperatures, it is not practiced.
Another limitation on the use of fluorocarbon polymers in conventional procedures is the fact that metal does not readily adhere to the surface. Thus, it is difficult to pattern electrical connections on a polytetrafluoroethylene surface.
Commonly assigned Cole et al., U.S. Pat. No. 5,073,814, issued Dec. 17, 1991, discloses use of a multi-layer composite of alternating thin layers of KAPTON polyimide (Kapton is a trademark of E.I. duPont de Nemours and Co.) and TEFLON polytetrafluoroethylene as a means to provide sufficient absorption of optical energy in the bulk of the layer to allow laser ablation. This process provides a lower dielectric constant than prior techniques while allowing for adhesion and laser drilling; however, it requires many repetitive depositions and does not provide all of the advantages of a pure form of polytetrafluoroethylene.
In high frequency applications, the use of high dielectric constant materials limits the present HDI process. Specifically, high dielectric constant materials (i.e., .epsilon. about 3.0) applied over chips contribute additional capacitance loading to the chips at high frequencies and thus alter the design performance of the chips. Improved propagation and reduced capacitive coupling are obtained with TEFLON polytetrafluoroethylene (.epsilon. about 2.0) because of its significantly lower dielectric constant. Additionally, video array chips need to have their viewing windows cleared of light blocking polymer materials to achieve proper optical response. Localized ablation has been used to clear polymers off of sensitive areas of chips; however, direct laser ablation on the chip surface sometimes damages the chip. Since TEFLON polytetrafluoroethylene is transparent down to 209 nm, this ablation damage can be reduced while still achieving optically clear layers.
Commonly assigned Eichelberger et al., U.S. Pat. No. 4,835,704, issued May 30, 1989, which is herein incorporated by reference, discloses a method for performing HDI adaptive lithography which allows unique processing of multi-chip modules with non-precision chip placement. The imprecision in placement is calculated by a computer and then written directly into photoresist without a mask using a CW argon ion scanning laser dither system.
Polymers that absorb light at wavelengths of approximately 350-360 nm work well with this CW argon ion laser system. The laser energy is absorbed by the polymer and locally heats the polymer to temperatures where thermal decomposition or ablation occurs. At these wavelengths there are other polymers, including fluorocarbon polymers such as polytetrafluoroethylene, which do not absorb incident energy and therefore cannot be readily ablated. Since polytetrafluoroethylene has essentially no absorption at wavelengths above 200 nm, both CW and pulsed excimer lasers at all wavelengths above 200 nm will not effectively ablate this material, thus limiting its use as a low dielectric constant interlayer dielectric for HDI adaptive lithography in which an adaptive laser is employed for via fabrication.
The process of via formation by adaptive lithography using laser dithering can cause extreme localized heating of the surrounding polymer and can put the polymer under stress and result in melting or flowing of the material. In the case of KAPTON polyimide, the stress caused by via dithering often results in the formation of a substantial polymer ridge around the surface perimeter of the via. Exposure of the via to excimer lasers or ashing procedures replicates the surface topography further down into the polymer, leaving the ridge intact. Subsequent metallization causes a much larger metal ridge to form around the via perimeter because of excessive electroplating in this area, due to high electric field density areas. Subsequent patterning of the metal around the via becomes difficult because of thin or incomplete resist coverage over the metal ridge, allowing etchants to attack the via metal. Additionally, the metal ridge can form shorts between metal levels, due to thinning of dielectrics.